1. Field of the Invention
This invention generally relates to the field of photolithography as employed for the fabrication of micro- and nano-structures, and it relates particularly to the field of Talbot imaging as employed for transferring periodic patterns of features defined in a mask onto a photosensitive layer on a substrate, and more particularly to the printing of patterns that are periodic in one dimension.
2. Description of Related Art
Lithographic fabrication enables the formation of micro- and nano-patterns on surfaces. Photolithographic techniques achieve this by exposing a photosensitive surface to a light-field with an intensity distribution corresponding to the desired pattern. The photosensitive surface is usually a thin layer of sensitive film, such as a photoresist, which is coated either directly onto a substrate or indirectly over intermediate layers of other materials or structured materials. Chemical or physical changes that occur in the photoresist is used in subsequent processes to obtain a desired pattern in the material of the substrate or in a layer of another material on the substrate. In the most commonly used photolithographic technique an image of the pattern, which is initially defined in a mask, is projected onto the substrate surface using an optical system.
Many applications require the formation of high-resolution grating patterns that comprise parallel lines and spaces that repeat with a sub-micron period in one direction. Examples of such applications are grating polarizers, coupling gratings for biomedical sensors, and light trapping gratings for solar cells. A specialized photolithographic technique based on the Talbot effect is advantageous for transferring such patterns from masks onto substrates because it avoids the use of a conventional imaging system which, for high resolution patterns, is generally complex and high-cost. In this technique a mask defining the periodic pattern is illuminated with a collimated beam of monochromatic light and the light diffracted by the pattern reconstructs “self-images” of the pattern at certain distances from the mask in Talbot image planes (see, for example, C. Zanke, et al., “Large area patterning for photonic crystals via coherent diffraction lithography”, J. Vac. Sci. Technol. B 22, 3352 (2004)). The separation, S, between successive Talbot image planes is related to the illumination wavelength, λ, and period of the pattern, p, byS≈2p2/λ  equ.(1)
Between the self-images in the Talbot planes are so-called Talbot sub-images that have higher spatial frequencies. By placing a photoresist coated substrate at one of these fractional Talbot planes, a periodic pattern is printed whose spatial frequency is higher than that in the original mask. The results achieved with this technique are improved when the duty cycle of the pattern in the mask, that is the ratio of the width of the lines to the width of the spaces, is optimized to yield a high-contrast intensity distribution in the fractional Talbot planes (see U.S. Pat. No. 4,360,586). In the prior art, it is also known that the contrast of the Talbot images can be further enhanced by fabricating the periodic patterns in the mask from phase shifting materials. With the Talbot technique, however, the intensity distributions of the self-images and sub-images are very sensitive to the distance from the mask, that is, they have a very narrow depth of field. This means the substrate has to be very accurately positioned with respect to the mask in order to obtain a correctly printed grating. This becomes increasingly more difficult as the grating period is reduced because the depths of field of the self-images and sub-images are proportional to the square of the pattern period. Furthermore, if the pattern needs to be printed onto a substrate that is not very flat or has topographical features on its surface, or into a thick layer of photoresist, it may be impossible to achieve the desired result.
Achromatic Talbot lithography has recently been introduced as a new method for printing high-resolution periodic patterns in a cost effective way (see N. Guérineau et al., “Talbot experiment re-examined: demonstration of an achromatic and continuous self-imaging regime”, Opt. Commun. 180, pp. 199-203 (2000); H. H. Solak, et al., “Achromatic Spatial Frequency Multiplication: A Method for Production of Nanometer-Scale Periodic Structures”, J. Vac. Sci. Technol., 23, pp. 2705-2710 (2005); and U.S. Pat. Appl. no. 2008/0186579). It offers two significant advantages for lithographic applications: firstly, it overcomes the depth-of-field problem encountered in the classical Talbot method described above, and secondly, for many pattern types the printed patterns have a higher spatial-frequency than that in the mask, that is, it can perform a spatial-frequency multiplication. Achromatic Talbot lithography (ATL) illuminates the mask with a collimated beam from a broadband source and the substrate to be printed is placed at or beyond a certain distance from the mask at which the image generated becomes stationary, that is, invariant to further increase in distance. The minimum distance, dmin, required for the stationary image to be formed is related to the period of the pattern, p, in the mask and to the spectral bandwidth of the illumination, Δλ, by:dmin≈2p2/Δλ  equ.(2)
The distance at which a particular desired level of insensitivity of the printed pattern to variation of distance between substrate and mask may be accurately determined by computer simulation. At this distance the Talbot image planes for the different wavelengths are distributed in a continuous manner with increasing distance from the mask, and so placing the substrate beyond this distance effectively exposes the substrate to the entire range of lateral intensity distributions that occur between successive Talbot planes for a particular wavelength. The pattern printed onto the substrate therefore corresponds to the integration, or average, of this range of transversal intensity distributions, and so is insensitive to further increase in distance from the mask. The resulting extended depth of field is also substantially greater than that of images formed using conventional lithographic techniques such as projection, proximity or contact printing.
If ATL is applied to one-dimensional, line/space patterns, the stationary image printed onto the substrate generally exhibits spatial-frequency multiplication: the period of the pattern is reduced by a factor of two. The intensity distribution in the ATL image produced by a particular mask pattern may be determined using modeling software that simulates the propagation of electromagnetic waves through masks, layers of other material and through space. Such simulation tools may therefore be used to optimize the design of the pattern in the mask for obtaining a particular printed pattern at the substrate surface.
The ATL method has been developed primarily to print periodic patterns that comprise a unit cell that repeats with a constant period in at least one direction. The technique may, however, also be successfully applied to patterns whose period spatially varies in a sufficiently “slow”, gradual way across the mask such that the diffraction orders that form a particular part of the stationary image are generated by a part of the mask in which the period is substantially constant. The tolerance to such variation in period may be determined using analytical methods or modeling software of the type mentioned above, and the patterns concerned may be characterized as being quasi-periodic.
A drawback of ATL arising from equ. (2) is that it requires a light source with a significant spectral bandwidth in order that the separation required between the mask and substrate is not disadvantageously large. The angular divergence of the different diffracted orders propagating from the mask produces spatial offsets between the orders at the substrate surface and therefore imperfect image reconstruction at the pattern edges, which becomes worse with increasing separation. Fresnel diffraction at the edges of the diffracted orders also degrades the edges of the printed pattern, which likewise gets worse with increasing separation. For these reasons laser sources, which have relatively small spectral bandwidth, are in most cases unsuitable for ATL.
A difficulty with applying non-laser sources, such as arc lamps or LEDs, to ATL is obtaining the combination of high power in the exposure beam (for ensuring high throughput in a production process) and also good beam collimation (for ensuring high-contrast Talbot imaging). Obtaining good collimation from these sources requires spatial filtering of the output beam which generally results in a large loss of power.
The advantages offered by the ATL technique may also be obtained using another prior art modification of the classical Talbot method. In this alternative scheme, the periodic pattern in the mask is illuminated by a well collimated beam of light and during exposure the substrate is displaced longitudinally relative to the mask by at least a distance corresponding substantially to the separation between successive Talbot image planes. The technique, which may be called Displacement Talbot lithography (DTL), also results in the substrate being exposed to the entire range of lateral intensity distributions between Talbot image planes, thereby also producing an integration, or averaging, of the entire range of transversal intensity distributions between Talbot planes over the duration of the exposure (see also U.S. patent application Ser. No. 11/665,323).
Whereas the integrated intensity distributions generated at the substrate by the ATL and DTL techniques are substantially the same, and both enable a large depth of field for the printed pattern and spatial-frequency multiplication, the DTL scheme has the advantage that it can be used with much smaller separations of the substrate and mask. This improves the edges of the printed pattern and allows higher utilization efficiency of the light source because of the less stringent requirement on collimation. Further, the DTL technique facilitates the use of laser sources, which generally provide substantially monochromatic light and are often preferred for production processes. Light from laser sources can be collimated well without loss of power, which enables a larger separation between the mask and substrate and also printing onto substrates that have significant warp or topography.
The structure of the patterns printed using DTL from a particular mask pattern can also be theoretically determined using simulation software. As for ATL, DTL is also not restricted to purely periodic patterns but may be applied to quasi-periodic patterns.
A drawback of the DTL technique is that the exposure equipment needs to provide a controlled displacement of the substrate with respect to the mask during the exposure, which can increase system complexity and cost, and can reduce reliability. This is especially true if the substrate is very large or if the substrate is exposed using a scanning strategy in which a beam of relatively small dimension is scanned across the substrate at high speed in order to achieve a short exposure time.
The as yet unpublished U.S. application Ser. No. 12/706,081 by the applicant describes refinements of the ATL and DTL techniques that employ a source with significant spectral bandwidth and a displacement of the substrate with respect to the mask respectively in order to print general periodic structures onto substrates with a large depth of focus. Because these refinements incorporate the principles of the ATL and DTL techniques for increasing the depth of focus of the printed patterns, they necessarily have the same drawbacks of those techniques.
It is therefore an object of the present invention to provide a method for printing onto a substrate a pattern of features that is periodic in one-dimension which provides a large depth of focus and does not require a relative displacement between a mask and substrate during the exposure.
It is a further object of the present invention to provide a method for printing a pattern of features that is periodic in one-dimension onto a substrate that enables the use of a laser source.
It is a specific object to provide a method for printing a high-resolution grating pattern onto large substrates in a short printing time.
It is a further object of the present invention to provide an apparatus for printing a pattern of features that is periodic in one-dimension such that the large depth of focus of the ATL and DTL techniques is obtained but without the above-mentioned associated disadvantages of these two techniques.
It is a further object of the invention for providing a method and apparatus for fabricating polarizer gratings, grating couplers for, for example, bio-sensors, and light couplers for solar cells.